This invention relates to a ZnS blue light emitting device of metal-insulator-semiconductor (MIS) structure having low-resistivity ZnS as luminescent layer and two or more composite insulating layers forming a high-resistivity layer for hole injection.
ZnS is a II-VI compound semiconductor with a band gap of 3.7 eV of direct transition type at room temperature and, like GaN, SiC and ZnSe, has been a subject of many researches as a highly efficient blue light emitting material. Monocrystalline ZnS grown by a method of high-pressure melting, sublimation or halogen transport is thermally processed in molten zinc at 850.degree.-1050.degree. C. after dopants such as Al or I are added and becomes low-resistivity n-type (.about.1 ohm-cm) with luminescence peak at 450-480 nm. Since p-type ZnS has not been obtained, use is currently made of light-emitting devices of MIS structure.
FIG. 3 shows a conventional light emitting device of MIS structure with a high-resistivity layer (I-layer) 3 formed on top of a low-resistivity ZnS layer (S-layer) 4 for hole carrier injection and an injection side electrode (M-layer) 2 further on top. An ohmic contact 5 is provided to the low-resistivity ZnS layer 4 and lead lines 1,1 are attached to the electrodes 2 and 5. The high-resistivity layer 3 may be an insulating layer formed by thermal processing of the low-resistivity layer 4, an insulating layer deposited by vapor deposition method or a high-resistivity ZnS layer formed by MOCVD (organic metal chemical vapor deposition) method. External quantum efficiencies of light emission as high as 0.08% have been reported.
With conventional light emitting devices of MIS structure, the I-layer 3 always consists of a single high-resistivity layer. Because of difficulties related to the characteristics of this layer controlling the properties of the device, its layer thickness, flatness, stability, etc., problems of stable operation of a light emitting device and its radiation efficiency have not been solved satisfactorily.